Free Bulge Test Research Articles

Improvement of material flow in tube hydroforming by advanced sealing methods

In this paper, two advanced sealing methods are proposed to enhance the formability of aluminium alloys in the tube hydroforming process. These methods are aimed at omitting friction force at the contact area between the tube and the die in the feed region and facilitating the material flow to the deformation region. The advanced sealing methods are numerically and experimentally examined against the conventional sealing method by conducting the free bulge test on AA6063 aluminium tubes. The deformation mechanics are deeply analysed in the principal strain space to identify the influence of the sealing methods on deformation paths. In addition, the effectiveness of the advanced sealing methods is evaluated under different lubrication conditions and contact lengths in the feed region. Results show that in the advanced sealing methods compared to the conventional one, the thickness in the feed region does not increase and the necking occurs in larger in-plane strains, leading to a higher bulge height. Results also allow concluding that a higher bulge height is formed using the second advanced sealing method, in which the tube is pressurized from both sides in the feed region. Thus, the advanced sealing methods are recommended for tube hydroforming of aluminium alloys with low formability.

Inverse Identification of a Constitutive Model for High-Speed Forming Simulation: An Application to Electromagnetic Metal Forming

Forming simulation requires a constitutive model whose parameters are typically determined with tensile tests assumed static. However, this conventional approach is impractical for high-speed forming simulation characterized by high strain rates inducing transient effects. To identify constitutive parameters in relation to high-speed forming simulation, we formulated the problem of constitutive modeling as inverse parameter estimation addressed by regularized nonlinear least squares. Regarding the proposed inverse constitutive modeling, we adopted the L-curve method for proper regularization and model order reduction for rapid simulation. For demonstration, we corroborated the proposed strategy by identifying the modified Johnson–Cook model in the context of a free bulge test with electromagnetic metal forming simulation. The L-curve method allowed us to systematically choose a regularization parameter, and model order reduction brought enormous computational savings. After identifying constitutive parameters, we successfully verified and validated the reduced and original simulation models, respectively, with a manufactured workpiece. In addition, we validated the numerically identified constitutive model with a dynamic material test using a split Hopkinson pressure bar. Overall, we showed that inverse constitutive modeling for high-speed forming simulation can be effectively tackled by regularized nonlinear least squares with the help of an L-curve and a reduced-order model.

Microstructure evolution of transformation induced plasticity seamless steel tube in hydraulic free bulge test

AbstractIn this study the hydraulic free bulge test with an internal pressure boosting velocities of 0.5 MPa/s and punch feeding speed of 0.2 mm/s was carried out to study the microstructure evolution of the transformation induced plasticity seamless steel tube in the hydroforming process. The microstructure of the selected parts of the tube after the hydraulic free bulge test were investigated with the optical microscope and transmission electron microscope. The transformation rate of the retained austenite to martensite of the selected parts were determined by the x‐ray diffraction. To study the deformation behavior and obtain the stress‐strain state of the tube, the finite element simulation of the hydraulic free bulge test was also carried out. The results show that the retained austenite in the microstructure of all the selected parts transformed to martensite after the hydraulic free bulge test. In the center part of the tube, the highest transformation rate of 82.84 % was obtained. The retained austenites distribute inside of the ferrite grains transform to martensite firstly under the compressive stress and strain state. The retained austenites distribute at the ferrite grain boundaries start to transforme into martensite when the tube is subjected to the tensile stress and strain.

Experimental Bulging and Buckling of Bi-segmented Cylinders

This paper studies the experimental free bulging and buckling of bi-segmented cylinders. Two cylindrical shells are manufactured using argon-arc welding and analyzed for manufacturing errors. Next, free bulging tests and hydrostatic pressure tests are conducted to verify the feasibility of the free bulging technique and to confirm the destruction form of the bulged bi-segmented barrel. At the same time, the reasonableness of the circular thick plate is examined, which is determined according to the deformation consistency. The results show that the bi-segmented cylindrical preforms exhibit good welding quality by TIG welding technology. It has the same critical pressure load as a single cylindrical preform. Nevertheless, the bi-segmented cylindrical preforms have larger internal volume and better buoyancy capability than the single cylindrical preform. The free bulging bi-segmented shell has good symmetry along the axial direction. It has larger internal volume and higher critical buckle capacities than the non-bulging one.

Biaxial formability and microstructure of an Al-Mg-Si alloy sheet post solution heat treatment

Hot forming-quenching integrated process was developed to fabricate aluminum alloy complex-shaped components by taking advantage of the enhanced formability at the temperature close to solution temperature. But, it is very difficult to evaluate the formability under biaxial tension state and continuous cooling conditions. A measuring method was proposed to evaluate the biaxial formability by combining solution heat treatment and hot gas free bulging test. The effects of temperature and pressurizing rate on the bulging ability and thickness uniformity of an Al-Mg-Si alloy sheet were clarified. The deformation mechanism was revealed by microstructure observation. It is found that excellent formability can be obtained under the high temperature with less heat loss and rapid pressurizing conditions. The limiting strain under rapid pressurizing condition decreases from 1.13 at 500 ℃ to 0.81 at 400 ℃, being at a high strain level. Thickness uniformity can be obviously improved at a higher deformation temperature accompanied by rapid loading due to the remarkable strain-rate hardening effect. Dynamic recovery is the main restoration mechanism, resulting in numerous substructures and unchanged grain size. The obtained biaxial formability closer to actual forming condition can provide fundamental guidance for forming aluminum alloy complex-shaped component by hot forming-quenching integrated process.

Analysis of the forming behaviors of magnesium alloy AZ31 by vaporizing metal foils

Vaporizing metal foils is a relatively new high-speed material processing technique which can improve the material’s forming limit and reduce the springback. This study aims to investigate the forming behaviors of sheet metals by vaporizing metal foils. A simple analytical model to calculate the energy efficiency of this forming method is firstly introduced. The forming behaviors of magnesium alloy AZ31 is analyzed by free bulging tests at room temperature. Besides, the mechanical behaviors of magnesium ally AZ31 is compared with that of aluminum alloy EN AW-6082. The experiments indicate that the magnesium alloy AZ31 exhibits good formability by vaporizing metal foils without heating treatment. Therefore, it is feasible to conduct plastic forming process of magnesium alloy ZA31 at room temperature, which is different from the traditional warm forming method for magnesium alloy.

Numerical simulation of superplastic bulge forming test

Superplastic blow forming is a technology of shell parts production. The development of these processes requires computer simulation which cannot be realized without accurate parameters of the applied material. This characterization can be based on results of free bulging tests. Characterization techniques utilize the models of the dome growth during the bulging test. This study is devoted to the assessing of friction coefficient effect on the linear behavior of normalized thickness - normalized height relation.

Determination of pressurizing rate during hot gas forming with integrated heat treatment of Al–Cu–Li alloy: deformation and strengthening behaviors

Hot gas forming with an integrated heat treatment was utilized to form complex-shaped components in one operation that obtain full strength. The effects of the pressurizing rate on the deformation and strengthening behaviors were critical. Free-bulging tests were conducted at different pressurizing rates (0.0088–2.37 MPa/s) to reveal the corresponding deformation and strengthening behaviors of an Al–Cu–Li alloy sheet and for choosing appropriate pressurizing conditions. The profile and thickness were analyzed to describe the deformation behaviors. The Vickers hardness under different pressurizing and deformation conditions was measured to evaluate the strengthening behavior. The microstructure evolution was observed to reveal the deformation and strengthening mechanisms. An excellent bulging ability was obtained under the full solution condition, with a maximum strain of 0.88 at a pressurizing rate of 0.034 MPa/s. The thickness distribution became more uniform under rapid pressurizing conditions because of the increasing strain and strain-rate hardening abilities. The hardness increased with increasing deformation and was not affected by the pressurizing rate. Dynamic recovery was the main deformation mechanism, and dynamic recrystallization occurred at a low pressurizing rate. Fine precipitates comprising the T1 phase (Al2CuLi, hexagonal structure) were the main strengthening phase. The pressurizing condition was only considered to obtain the maximum improvement in formability and microstructure, regardless of its effect on strengthening during hot gas forming with an integrated heat treatment.

Superplastic Materials Characterisation Based on Free-Bulge Tests

A new characterisation method based on free-bulge tests is proposed to find the characteristic parameters of the strainstress relation in superplastic alloys. The method is applied to experimental tests found in the literature and to new adhoc tests performed on aluminium, magnesium and titanium alloys. The parameters are then compared to independent values obtained from tensile test and computer-aided simulations. The characterisation of the material is evaluated as an apparent viscosity function of the strain-rate and is extracted from the height evolution at the apex dome during the forming process. Results show a good agreement between parameter estimation using this method and independent values. Moreover, this characterisation method exposes the non-newtonian fluid behaviour of these materials during superplastic processes known as shear-thinning or pseudoplasticity.

A forming time estimator of superplastic free bulge tests based on dimensional analysis

Dimensional analysis is performed as a method to evaluate the characteristics of superplastic forming processes. The analysis is focused on the forming time results from superplastic free bulge tests so that an estimator for the forming time is obtained based on dimensionless parameters. The dimensional analysis is performed by applying the normalisation to the dynamic equations and their corresponding boundary conditions, from which five dimensionless parameters are obtained. Particular conditions of the tests allow to reduce the parameters to two. This preliminary study of the applicability of the dimensional analysis on superplastic forming processes will guide for further steps in which this technique may help during the initial stages of the process layout.

Influence of Combined Severe Plastic Deformation and Sheet Extrusion Process on the Superplastic Formability of AA 5083 Aluminum Alloy Assessed by Free Bulge Test

A combination of two forming operations is considered for producing metal sheets with improved mechanical properties. As the first operation, a newly introduced severe plastic deformation method called dual equal channel lateral extrusion (DECLE) was performed at 300 °C for different passes on the AA 5083 aluminum blocks. Following the DECLE operation, sheet extrusion was conducted to convert the bulk samples, severely deformed through various passes, into 1.8-mm-thick sheets. Mechanical properties of the processed specimens, after each step of deformation, were examined using tensile and shear punch tests. It was found that the material after three passes of DECLE and also the corresponding forwardly extruded sheet presented the greatest strength. In order to evaluate the biaxial formability of the sheets, gas bulge forming tests were conducted using a PLC-controlled gas circuit. It was shown that the material processed via three passes of DECLE operation and extrusion demonstrated the maximum biaxial superplastic formability with an effective strain associated with 400% uniaxial elongation. This sheet specimen also required the minimum forming time, coinciding with a strain rate of 4 × 10−4 s−1, which is higher than strain rate for the sheet extruded from the annealed sample.

Acquisition and Evaluation of Theoretical Forming Limit Diagram of Al 6061-T6 in Electrohydraulic Forming Process

The current study examines the forming limit diagram (FLD) of Al 6061-T6 during the electrohydraulic forming process based on the Marciniak–Kuczynski theory (M-K theory). To describe the work-hardening properties of the material, Hollomon’s equation—that includes strain and strain rate hardening parameters—was used. A quasi-static tensile test was performed to obtain the strain-hardening factor and the split-Hopkinson pressure bar (SHPB) test was carried out to acquire the strain rate hardening parameter. To evaluate the reliability of the stress–strain curves obtained from the SHPB test, a numerical model was performed using the LS–DYNA program. Hosford’s yield function was also employed to predict the theoretical FLD. The obtained FLD showed that the material could have improved formability at a high strain rate index condition compared with the quasi-static condition, which means that the high-speed forming process can enhance the formability of sheet metals. Finally, the FLD was compared with the experimental results from electrohydraulic forming (EHF) free-bulging test, which showed that the theoretical FLD was in good agreement with the actual forming limit in the EHF process.

Investigations of sheet metal forming by vaporizing metal foils

This study aimed to investigate the forming behavior of magnesium alloy by vaporizing metal foils. This process was conducted at room temperature and no heat treatment for the workpiece was required. The free bulging test was applied to identify the formability of the magnesium alloy AZ 31, and the perforated sheet forming test was used to measure the pressure distributions. The effectiveness of the polyurethane plates on generating shock pressure for the metal forming work was firstly examined. The pressure distributions of vaporizing single foils and tailored foils were subsequently discussed. The pressure distributions were mainly determined by the active areas of the foil specimens. AZ 31 magnesium sheets with a thickness of 1 mm and 1.8 mm were formed by the free bulging test under different charging energies. A bulging height of 27.28 mm for the AZ 31 sheet in 1.8 mm was acquired under a charging energy of 4.8 kJ. Vaporizing foils was found to be a feasible technique to conduct magnesium alloy forming without heat treatment for the workpiece.

Pressure distributions generated by vaporizing metal foils

Abstract The pressure distributions generated by vaporizing metal foils were studied. An analytical model which described the dynamic mechanical behavior of a rectangular plate under an impulsive loading was introduced. The formed parts of free bulging tests were analyzed using the optical measurement system. Two measurement methods for pressure distributions were introduced and compared. Both the perforated sheet forming test and the pressure film were found to be effective method to measure pressure distributions. The cost of perforated sheet forming test was cheap and the pressure film was easy to operate. Three different pressure distributions were measured and discussed, namely single pressure distribution, tailored pressure distribution and double-direction pressure distribution. These three pressure distributions could be applied in different metal forming processes.

Characterization of Stress-Strain Behavior of Superplastic Titanium Alloy by Free Bulging Tests with Pressure Jumps

The work is dedicated to determination of stress-strain behavior of Ti6Al4V alloy deformed in conditions of biaxial tension provided by free bulging testing. The dome height during each test was continuously measured and recorded using a magnetostrictive position transducer. All the tests were performed using stepped pressure regime with jump pressure changing between two values at evenly spaced time moments. This experimental technique provides the possibility to study strain rate sensitivity index variation during the test and subsequently construct strain and strain rate dependent material model. The output data of each test include the evolution of dome height, subsequent pressure regime and final thickness of the specimen at the dome pole. In the framework of this study the processing of such data in order to evaluate the material behavior is discussed. Inverse analysis with different material models was implemented as well as special direct technique allowing one to construct stress-strain curves based on the results of free bulging tests with pressure jumps. The obtained material model was verified by finite element simulation.

Determination of tubular material parameters in bulging test with three-dimensional digital image correlation method

Tubular material parameters play a major role in part manufactory process planning and reliable finite element simulations. The hydraulic bulging test is one of the most credible ways to determinate tubular material properties, especially flow stress. In this paper, an on-line measurement method based on three-dimensional (3D) digital image correlation method (DIC) is applied to determinate material parameters of tube using Swift law. Both a practical bulging machine and a self-developed measurement system are introduced. The free-bulging test is performed for AlMg2.5 aluminum alloy tube. The axial curvature radius is calculated by extracting the bulging profile based on the cosine model and the axial strain is computed accurately by tracking a series of points around the pole dome. Finite element simulations on tube are conducted to validate the method. The results indicate that the material parameters obtained from the present method are reliable to define the flow stress in the tube hydroforming process.

Tailored pressure distributions generated by vaporizing tailored metal foils

This study aimed to realize tailored pressure distributions by vaporizing metal foils. The foil specimen with two separate active areas can be fully vaporized at both tailoring positions using capacitor bank discharge. The shock pressure amplitude was revealed by free bulging test. The length effect of the foil specimen on generating shock pressure for the metal-forming work was firstly examined. Aluminum foil with an active length up to 300 mm was completely vaporized under a charging energy of 6 kJ. Using different combinations of process parameters including foil geometry and polyurethane plates, a variety of tailored pressure distributions were created. The pressure amplitudes at two separate positions can be varied by changing the process parameters used such as foil length and width and polyurethane thickness. Finally, the tailored pressure distribution was applied to sheet metal-forming process and a part with different forming depths was manufactured.

High temperature deformation behavior of friction stir welded 2024-T4 aluminum alloy sheets

The deformation behavior was characterized using uniaxial tensile and free bulging testing. Digital Image Correlation (DIC) system was used to probe the full strain fields during the whole bulging process. Thickness distribution of the friction stir welded (FSW) sheet was analyzed and compared with the base metal. The flow stress of FSW joint was lower than that of base metal at the temperature of 400°C and initial strain rate of 0.001s−1. The ultimate tensile strength of FSW joint was about 70% of the base metal. The deformation was concentrated in the nugget zone of the weld during both uniaxial tensile tests and free bulging tests at high temperature, exhibiting significant inhomogeneous deformation characteristic. The bulging height of FSW sheet was only 50% of the base metal. The directions of major strains of weld zone and adjacent base material in the FSW sheet were vertical to each other during bulging tests. Such an incompatible deformation behavior resulted in the poor formability of FSW sheet compared with base metal.

Investigation of stress-strain behavior of a sheet material using free bulging test

Abstract In this work, a new experimental-numerical technique is developed in order to investigate the constitutive behaviour of a sheet material in conditions of superplastic forming. The principal feature of this technique is that unlike classical tensile testing it allows one to obtain stress-strain curves for a material formed in biaxial tension conditions produced by free bulging process. These conditions are much closer to the ones that the material undergoes during the superplastic forming process. Consequently, they give more accurate information about the material behaviour than the ones coming from tensile tests data. The drawback is that the strain (and similarly its time derivative) cannot be directly measured and controlled during free bulging test but its value has to be derived from other macroscopic measurement. Towards this end, a blow forming machine was equipped with a position transducer for the measurement of the dome height during the test. In order to control the stress in the dome apex at a predetermined level the applied pressure was continuously adjusted to current dome height using a special algorithm. After the test, the dome height data were processed to obtain the evolution of stress, strain and strain rate at the dome apex as well as the stress strain curves for constant referenced strain rates. The tests were performed on superplastic aluminium alloy (ALNOVI-U) sheets of 1.35 mm initial thickness at 500 °C. Using the data from two tests with different strain rate paths the stress-strain curves and the strain rate sensitivity index evolution were calculated for two constant referenced strain rates. The obtained constitutive data were verified by finite element simulation of a blow forming.

Characterization of Superplastic Materials by Results of Free Bulging Tests

Determination of material constants describing its behavior during superplastic gas forming is the main subject of this study. The main feature of free bulging tests is the stress-strain conditions which are very similar to ones occurring in the most of gas forming processes. On the other hand, the interpretation of the results of such tests is a complicated procedure. The paper presents a simple technique for the characterization of materials superplasticity by free bulging tests, which is based on inverse analysis. The main idea of this technique is a semianalytical solution of the direct problem instead of finite element simulation which allows one to reduce the calculation time significantly. At the same time the results this simplified solution are accurate enough to obtain realistic material constants.